This invention relates to antennas and more particularly to broadband horn antennas.
The conical corrugated horn antenna, known also as the scalar horn antenna, has an inner surface formed with coaxial axially spaced corrugations or slots which produce a far field circularly symmetrical constant beamwidth. The useful bandwidth of such a horn, however, is approximately 1.7:1 which limits its application. For example, there are microwave receivers currently available which may be tuned over frequency ranges of 8-12 GHz and 12-18 GHz, respectively, so that two such receivers may be employed in tandem to cover the 8 to 18 GHz band. It is advantageous for many reasons to have two such receivers share a single antenna but to accomplish this the antenna must have an operating bandwidth of at least 2.25:1, i.e., it must have acceptable performance characteristics over this band.
Efforts to extend the bandwidth of the corrugated horn antenna have included forming the horn with broadband slots such as partial dielectrically loaded slots, tapered slots, or ridge loaded slots, the latter being described in a paper entitled "The Ring Loaded Corrugated Waveguide" by Y. Takeichi et al published in IEEE Transactions on Microwave Theory and Techniques, December 1971, pages 947-950. While such horn constructions have resulted in some bandwidth improvement, the radiation pattern nevertheless still deteriorates at the upper end of 8 to 18 GHz band so that the antenna is unacceptable for use in high performance receiving systems operating over this band.
A technique for increasing the bandwidth of the corrugated horn antenna to cover the 8-18 GHz range is described in the copending application of Craig Roberts and Samuel Kuo, Serial No. 691322, assigned to the assignee of this application, and consists generally of suppressing higher order conventional modes produced in the feed waveguides which ultimately cause pattern deterioration. This technique includes the use of mode suppressors in the input feed waveguide for horns having broadband corrugations and which propagate into open space. There are applications, however, which because of the environment in which the horn must operate require the use of a radome on the horn in order to mechanically seal the antenna system. The difficulty with use of a conventional radome over the aperture of the horn is that it distorts waves propagating through the radome wall at some frequencies in the 8-18 GHz operating band producing side lobes and narrowing of the radiation pattern, especially in the E-plane. Such adverse effects on antenna performance have precluded use of radomed horns over this frequency range.